The present invention relates to methods and apparatus for generating and using electrical signals with precisely specified and controlled angle noise.
In general, any signal is characterized as having an amplitude and an angle. Any disturbance of the angle of a signal is referred to as angle disturbance. The angle of a signal includes a frequency term, a time term and a phase term. Any disturbance of the frequency term is referred to as frequency noise, any disturbance of the phase term is referred to as phase noise and any disturbance of the time term is referred to as time noise (time jitter). Frequency noise, phase noise and time noise are forms of angle disturbance.
Noise, and particularly phase noise, is a significant deleterious factor for many signals employing various modulation formats used in the communication industry. Phase noise causes the actual signal phase of a generated signal to deviate from the ideal phase of the generated signal. Typically, phase noise in communication systems is a random phenomenon that has higher amplitude at lower noise frequencies than at higher noise frequencies.
Many types of signal modulation formats are adversely affected by noise including, by way of example, Phase Modulation (PM), Frequency Modulation (FM), Phase Shift Keyed (PSK) Modulation and Quadrature Amplitude Modulation (QAM), Quadrature Phase Shift Keyed (QPSK) Modulation, Frequency Shift Keyed (FSK) modulation, Frequency Hopped (FH) modulation and Ultra Wide Band (UWB) modulation.
In general for new communications systems that attempt to transmit more data for a given bandwidth, phase noise significantly affects signal quality and system performance. In order to transmit more data for a given bandwidth, various formats such as high-order PSK or QAM are employed and these formats require low phase noise. Since these formats are often used in an environment of tightly packed channel spacing, the requirements for low phase noise are even more important.
There are many techniques known for reducing phase noise in signal-generating devices. Typical examples are described in U.S. Pat. Nos. 6,072,371, 6,175,284 and 6,236,275. U.S. Pat. No. 6,072,371 employs heterojunction circuit designs to reduce phase noise. U.S. Pat. No. 6,175,284 employs temperature compensated crystal circuit designs to reduce phase noise. U.S. Pat. No. 6,236,275 employs digital frequency synthesis to reduce phase noise. None of these techniques contemplates imparting a specified and known phase noise on the generated output signal.
There are many techniques known for measuring phase noise. Typical examples of methods known for measuring phase noise are described in U.S. Pat. Nos. 6,167,359 and 6,313,619. U.S. Pat. No. 6,167,359 employs non-linear, differential-equation analysis for characterizing phase noise. U.S. Pat. No. 6,313,619 uses a spectrum analyzer for phase noise measurement. None of these known techniques, however, contemplates controlling or generating a signal with specified and accurate phase noise.
A spectrum analyzer is the most common device for measuring phase noise. Typically, a spectrum analyzer includes modules to automate phase noise measurement and operates on a sinusoidal input signal. To measure xe2x80x9ctruexe2x80x9d phase noise, measurement systems should perform a phase demodulation of a generated signal and then measure the power spectrum of the demodulated signal. Such measurement systems, however, are difficult to build and calibrate. Therefore, measurement systems typically assume the phase noise is small and make the small angle approximation, sin(x)=x and cos(x)xe2x89xa11. This approximation means the spectrum of the phase noise is approximately the same as the spectrum of the generated signal itself minus the desired tone for a sinusoidal signal. If the phase noise is large, however, this approximation is significantly inaccurate and these measurement systems produce erroneous results. Measured phase noise results are typically plotted in log power (dBc/Hz) versus log frequency.
One well-known method for producing a signal with specified phase noise operates by generating a phase signal and then applies that phase signal to the phase-modulation input of a standard signal generator. This method is described, for example, in the publication xe2x80x9cAudio Noise Sources for Generating Phase Noisexe2x80x9d by Charles Wenzel, available at the WEB address xe2x80x9chttp://www.wenzel.com/pdffiles/noise.pdfxe2x80x9d as of the date of this application. This method and other known methods are accurate at best only to about +/xe2x88x921 dB to 2 dB and even to achieve such poor accuracy, these methods are difficult to calibrate. Such poor accuracy and such calibration difficulty are unsatisfactory to meet the needs of communication systems and devices in the communications industry.
Accordingly, in order to meet the needs of the communication industry, improved methods and apparatus for generating and using signals with exact, accurate and specified noise are required.
The present invention is a digitally controlled angle noise signal generator for generating a signal with precise and accurate noise. The signal generator is formed of a digital noise generator for generating a digital noise signal xcex8N, and a digital signal generator for forming an internal signal S. The digital signal generator receives the digital noise signal xcfx86N and modulates the internal signal S with the digital noise signal xcfx86N to generate a digitally controlled signal yg=S{xcex8N}, where  is the modulation operator, with precise and accurate noise.
In embodiments of the invention, the digital noise generator generates the digital noise signal xcex8N as a phase noise signal, as a frequency noise signal or as a time noise signal.
In some embodiments, the internal signal S is a sine wave and in other embodiments the internal signal S is a modulated signal modulated with a modulation format. Typically, the modulation format is one of the modulation formats Phase Modulation (PM), Frequency Modulation (FM), Phase Shift Keyed (PSK) Modulation and Quadrature Amplitude Modulation (QAM), Quadrature Phase Shift Keyed (QPSK) Modulation, Frequency Shift Keyed (FSK) modulation, Frequency Hopped (FH) modulation and Ultra Wide Band (UWB) modulation.
In some embodiments, the digital noise generator generates the digital noise signal xcex8N as a phase noise signal and digital generates white noise. The white noise is digitally filtered to produce the digital noise signal xcex8N with precise and accurate noise. In one embodiment, the digital filtering is with an infinite impulse response (IIR) filter and in another embodiment, the digital filtering is with a finite impulse response (FIR) filter.
The digitally controlled signal yg is is further processed in an output processor. In one embodiment, the output processor includes a digital-to-analog converter that forms a converted signal and the converted signal is filtered to provide a filtered signal having accurate and controlled noise.
In some embodiments, the filtered signal has accurate and controlled noise at a first frequency and the first frequency is up-converted to an RF frequency to form an RF signal with precise and accurate noise.
The digitally controlled angle noise signal generator is used in a calibration apparatus for calibrating a noise analyzer. The digitally controlled signal yg is connected as an input to the noise analyzer to cause an analyzed digital signal to be produced in the noise analyzer. A comparator compares a digital signal from digitally controlled angle noise signal generator with the digital noise signal xcex8N to determine errors in the analyzed digital signal.
The digitally controlled angle noise signal generator is used for testing a communications system. The digitally controlled signal yg is connected as an input to the communications system. In one embodiment the input is to the local oscillator of the communications system. In another embodiment, the digitally controlled signal yg is in put to an RF receiver port of the communications system.
In some embodiments, the digital noise generator generates the digital noise signal xcex8N as a phase noise signal and executes an Inverse Fast Fourier Transform of a digital function to produce the digital noise signal xcex8N.